Method of filter media manufacture



April 26, 1966 E. v. PAlNTER ETAL 3,248,465

METHOD OF FILTER MEDIA MANUFACTURE Original Filed June 1l, 1959 Pinar-Ep/N SI1-u United States Patent O 8 Claims. (Cl. 26d- 113) Thisapplication is a division of our original application, Serial No.819,757, filed June 11, 1959, and now abandoned.

Ths invention relates to a method of manufacturing filter media, andmore particularly to the method of manufacturing a boil-proof, bonded,omnidirectionally strong filter media having graded porosity. The mediais formed by air depositing a heterogeneous mixture of fibers andthreads and through-bonding the fibers and threads with a reactionproduct of polyvinyl alcohol and a polycarboXylic acid, reacted in situ.

Various filter media have been used heretofore for the separation ofsolid material from iiuid material, e.g., for the filtration of milk. lnpresent-day practice wherein a so-called parlor milking system is used,the milk from a num-ber of cows is delivered to a refrigerated holdingtank through a pipeline system, and it is desirable to filter the milkfrom each cow before it is delivered into the main pipeline. The filtersfor this purpose are called in the line filters. In such a system, thefilter material must withstand considerable direct pressuredifferentials as well as sudden surges of reverse pressures. Inaddition, it must be able to withstand boiling temperatures in instanceswhere the milking system is sanitized by Very hot aqueous solutions. Theart is confronted by the problem of providing economical and efficientfilter media having these desirable strength properties, and alsoadequate filtration etiiciency. in addition, the art is -confronted bythe problem of providing a filtration media having a white surface onwhich large `dirt particles may be quickly or readily observed, eg.,through a transparent filter holder or unit.

The discoveries associated with the invention and relating to solutionof the above problems, and the objects achieved in accordance with theinvention as set forth herein include the provision of a method ofmaking boilproof lter media having -omnidirectional strength and gradedporosity which method comprises moving a web of a mixture having thefollowingcomposition:

2O parts by weight yof crimped rayon of about 1.6 inch staple length andabout 5 5 denier (15.5 micrograms per inch actual fiber length)diameter, each fiber of which contains 6 to 15 crimps per inch (crimpedbasis) each crimp having an amplitude (height from mean) of about V32inch and a wave length of 1/15 to 1/6 inch;

45 parts by weight of similar crimped rayon of a fineness of 8.5micrograms per inch;

20 parts by weight of similar crimped rayon of a fin-eness of about 4.25micrograms per inch;

parts by weight of cotton having a fineness of about 3 to 6 microgramsper inch (9.9 to 14 microns diameter average) and an average classerstaple length of about 0.8 inch; and

parts by weight (based on the weight of the above mixture) of bleachedabsorbent cotton or rayon threads 0.5 to 3 inches in length, and havinga cotton count of 40; into a flowing air stream substantially uniformlyacross its width, and stopping the movement of' the fibers by aforarninous barrier while continuing the movement of the air, so as toplace the heavier fibers at the bottom and ICC the remaining fibers insuccessive zone thereon according to their fineness, said fibers beingarranged at random in a web transverse to the predominant direction ofthe air How, separating the resulting web from the air stream,impregnating said web with an aqueous solution containing polyvinylalcohol and a polyacrylic acid and having a pH in the range of about 3.0to 5.0, the wet pick-up of the solution being in the range of 200 to300% and placing it in an oven at 250 to 400 F. for 2 to 4 minutes; theprovision of a method wherein prior to the impregnation there is placedupon the random directional web a layer of crimped rayon of 3 to 8`denier and 6 to l5 crimps per inch, the weight of the saidsuper-imposed layer being about 200 to 800 grains per square yard andthe weight of the `random directional layer being about 600 to 1200grains per square yard; the provision of a method wherein the addedlayer is of 3.0 denier rayon having 9 to 12 crimps per inch, said layerweighing 400 grains/sq. yd.; the provision of a method wherein theweight of the random directional layer is 900 grains/sq. yd.; theprovision of boil-proof non-woven filter media composed of cellulosicfibers bonded -by the reaction product of polyvinyl alcohol and apolycarboxylic acid of at least 90,000 molecular weight in situ, thepercentage of binder on a dry solids basis being in the' range of 1.25to 10.6, said fabric having a minimum tensile strength after immersionin boil-- ing water for 20 minutes of at least 0.2 lb./in., theproportion of the alcohol being in the range of 1 to 3 parts per part byweight of the polycarboXylic acid; the provision of such material havingomnidirectional strength and a boiled test strength of at least 1.0lb./in. width; the provision of such material having graded porosity;the provision of such material having a relatively coarse incomingfiltering layer followed by `a relatively fine filter layer, saidincoming layer being relatively smooth and white so as to provide aninspection surface; the provision of such material wherein the incominglayer is carded crimped rayon of 3 to 8 denier having 6 to 1 5 crimps-per inch; the provision of such material wherein the incoming layer isfollowed by a graded pore zone of the following composition;

20 parts by weight of crimped rayon of about 1.6 inch staple length andabout 5.5 denier (15.5 micrograms per inch actual fiber length)diameter, each fiber of which contains `6 to l5 crimps per inch (crimpedbasis) each crimp having an amplitude (height from mean) of about 132inch and a wave length of 1/15 to 1/6 inch;

45 parts by weight of similar crimped rayon of a fineness of 8.5micrograms per inch;

20 parts by weight of similar crimped rayon of a fineness of about 4.25micrograms per inch;

15 parts by weight of cotton having a fineness of about 3 to =6micrograms per inch (9.9 to 14 microns diameter average) and an averageclasser staple length of about 0.8 inch; and 20 parts by weight (basedon the weight of the above mixture) of bleached absorbent cotton orrayon threads 0.5 to 3 inches in length, and having a cotton count of`40; the provision of such material being in the shape of a tube formedby rolling and partially overlapping a sheet, said overlap being bondedby a boilproof polyethenyl adhesive, and other objects which will beapparent as details or embodiments of the invention are describedhereinafter.

In connection with this disclosure, reference will be made to theaccompanying draw-ing wherein FlG. l is a schematic cross-sectional viewof a filter medium made in accordance with the invention. FIG. 2 is aschematic end view of such material rolled up to form a tube with anoverlap seam. FIG. 3 is a schematic enlarged crosssectional view of theoverlap of such a tube, showing an adhesive inter-layer, and FIGURE 4 isa schematic crossr v sectional view of apparatus illustrating theformation of a fiber web of graded porosity. In more detail in thesefigures, indicates the media generally, 11 indicates a bottom layer madeup of coarser zone 11b and a finer zone 11a, and 12 indicates a cardedor isotropic layer which is coarser than zone 11a and 13 indicates aheatsealable adhesive such as polyethylene. In order to facilitate aclear understanding of the invention, the following preferred specificembodiments are described in detail.

-EXAMPLE 1a A composite filter sheet is made up in the following manner.

A mixture or carded web of the following composition is used in thefirst step:

20 parts by weight of crimped rayon of about 1.6 inch staple length andabout 5.5 denier (15.5 micrograms per inch actual fiber length)diameter, each fiber of which contains 6 to 15 crimps per inch (crimpedbasis) each crimp having an amplitude (height from mean) of about 1/32inch and a wave length of 1/15 to 1/6 inch;

45 parts by weight of similar crimped rayon of a fineness of 8.5micrograms per inch;

20 parts by weight of similar crimped rayon of a fineness of about 4.25micrograms per inch;

parts by weight of cotton having a fineness of about 3 to 6 microgramsper inch (9.9 to 14 microns diameter average) and an average classerstaple length of about 0.8 inch; and

parts by weight (based on the weight of the above mixture) of bleachedabsorbent cotton or rayon threads 0.5 to 3 inches in length, and havinga cotton count of 40, which cotton count is defined as the number ofyards in one pound of thread, divided by 840.

The card or web of this mixture is rapidly moved transversely into agenerally horizontal flowing air stream, individual fibers or threadsthereof being suspended in this stream substantially uniformly acrossits width, and then the movement of the fibers is stopped by aforaminous barrier while continuing the movement of the air, so as toplace the fibers at random in a web arranged transverse to thepredominant direction of the air iiow, and the resulting web isseparated from the air stream. Suitable apparatus for this operation isdescribed in U.S. Patents 2,676,363 and 2,676,364.

In this operation, the card web 13 is fed into the apparatus asillustrated in FIGURE 4, FIGURE 4 illustrating apparatus similar, forexample, to that described and illustrated in U.S. Patent No. 2,676,364.The card web is broken up by the licker-in 14 which rotates in thedirection to drive the fragments or fibers 15 in a downwardly direction.The fibers or fragments are carried generally horizontally by the airflow to the foraminous barrier 16 which is in the form of a cylinderrotating so that the surface contacted by the fibers is moving in anupwardly direction. The heavier or larger fibers, threads or fragmentstend to go into the lower region or zone of the moving air stream, andthus are the first to be deposited on the foraminous barrier. The lesscoarse or heavy fibers remain in a zone which is above theabovementioned zone and thus are deposited just above theabove-mentioned layer. In this way, there is built up a composite on theforaminous barrier having a gradient of fiber thickness, oralternatively, of porosity.

There is superimposed upon the above-described web a carded web of 3.0denier crimped rayon of the abovedescribed type. In one modification,the upper carded web weighs 400 grains per square yard and the lower webweighs 900 grains per square yard.

A bath is prepared containing 1.18% by weight of polyvinyl alcohol(Elvanol 72-60, i.e., high viscosity, substantially 100% hydrolyzed) and1.18% of cross-linked polyacrylic acid (prepared from acrylic acidmonomer, a small proportion of allyl sucrose, potassium persulfatecatalyst and water in accordance with Example 2 of U.S.

Patent 2,798,053; or Carbopol 934 polycarboxylic acid) in water. The pHof the bath is adjusted to about 3.3 by adding phosphoric acid (e.g.,0.0121% of strength phosphoric acid). The above-described compositesheet is passed through the bath at about F. and squeezed to removeexcess solution and leave a pick-up of 250% of the solution based onfabric weight. The resulting impregnated sheet material is dried for 210seconds in an oven at 300 F.

The composite web is passed in a continuous manner through a bondingunit and then through a drier. The drying or setting is at a temperatureof about 250 to 400 F.

Samples prepared in accordance with this procedure, when submerged inboiling water for 20 minutes, give wet tensile strength tests of 4.0lbs/in. width in the major fiber direction (i.e., for a carded material)and 1.7 lbs/in. in the cross direction, for a material weighing 3.0oz./yd.

A sample made in accordance with the above procedure, when soaked for1/2 hour in boiling 10% caustic shows no visible degradation orswelling. Similar samples soaked in 10% hydrochloric acid at roomtemperature for 1/2 hour also show no visible degradation or swelling.

These results are indeed surprising, especially in view of the testexperience with viscose bonded fabrics which lack the desired porosity.Fabrics of the above type have a low load density range of 0060-0150grams/cc., whereas if viscose bonded, they have a low load density rangeof 0.19-0.28 grams/cc. which is too compact for adequate filtration.Fabrics with boil-proof properties produced with certain latices such asthe vinyls and acrylics contain extractable surface active agents.Similarly, such fabrics produced from formaldehyde containing resinscontain extractable material to a more pronounced degree than materialsmade in accordance with this invention; and, the acid and alkaliresistance of the products of the present invention is superior to suchfabrics.

Any desired type of polyvinyl alcohol of any convenient viscosity suchas l5 to 75 centpoises (preferably 28 to 65 centipoises in 4% solutionat 20 C.) may be employed in the practice of this invention. Polyvinylalcohol is generally produced by hydrolysis of polyvinyl acetate andeither completely hydrolyzed or partly hydrolyzed products may be used.

EXAMPLE 1b The above-described product may be used in any one of variousforms. In one form it is cut into disks, e.g., 4%6" in diameter, andused in an in the line filter in a parlor milking system. It givesdesirable filtration and has sufiicient strength to withstand both thedirect iiow and also the occasional reverse flow pressures. In addition,any large dirt particles present are readily observable on the whiteinspection surface (which is the incoming surface).

EXAMPLE 1c In another modification, the above-described product may becut into sheets, e.g., 6% x 6%, rolled up into a tube with doubleoverlap at the seam, inserted in a tube holder and used in an in theline filter in a parlor milking system. It gives desirable filtrationand has sufficient strength to withstand both the direct fiow and alsothe occasional reverse flow pressure. Any large dirt particles presentare readily observable on the white inspection surface (which is theincoming surface).

EXAMPLE ld In another modification, the above-described product may becut into sheets, e.g., 6%" by about 5%" and rolled up into a tube withabout a 1/2" overlap, adhered at the overlap by insertion of a sheet ofhigh density polyethylene, the overlap being heat-sealed atv sufiicienttemperature to melt or soften the adhesive polyethylene, Withoutscorching or otherwise harming the bonded fabric, e.g., at a temperatureof 375 F. It gives desirable filtration and has sufficient strength towithstand both the direct flow and also the occasional reverse flowpressures. Any large particles present are readily observable on thewhite inspection surface (which is the incoming surface).

EXAMPLE 2 The above example is repeated except that the carded web is of5.5 denier rayon and the bath contains 1.26% of the alcohol, 0.63% ofthe polyacrylic acid and 0.0289% of phosphoric acid (of 75% strength).Similar results are obtained.

EXAMPLE 3 The above procedure is repeated except that the bath consistsof 2.36% of the polyvinyl alcohol, 0.79% of the polyacrylic acid and0.0242% of phosphoric acid (75% strength), and the solution pick-up isabout 250%. Similar results are obtained.

Thickness (from which density is calculated) is measured under a LowLoad compacting force of 0.16 p-.s.i. The maximum pore diameter ismeasured by the bubble point technique (Lordv and Taylor, I. Text.Inst., vol. 45, No. 5 (May 1954) p. T371), while the sample is heldunder a compacting force of 0.053 p.s.i., 15 p.s.i. and 30 p.s.i. Thewash resistance is measured in terms of the gal/min. flow rate of astream of water from a Mi diameter tube 2 inches away from the samplewhich is just able to rupture the sample.

Comparable results to the foregoing may be achieved with variousmodifications thereof including the following. The fibers should be atleast 1 mm. in length and preferably of a convenient length to behandled by conventional machines, which give an isotropic or randomarrangement of the fibers. Also, the longer fibers tend to give productshaving better wet-strength. Of the natural cellulosic materials, thewood pulp fibers may be in the range of about 1 to 5 mm. in length, andthe usual textile fibers may be in the range of about 1 to 5 cm. or morein length. There is no fixed upper limit for the fiber length, except,of course, the size can be handled on the carding or other machine, orthe size of the nished material- The fiber fineness is convenientlyexpressed in terms of micrograms per linear inch, as known in the art(as determined by weighing measured lengths of fibers or by using knowninstruments, e.g., a commercially available instrument) the actualequivalent average diameter may be determined from this value and thedensity of the fiber.

Cellulosic materials are economically available in abundant supply, andare preferred for economic reasons, e.g., cotton, viscose rayon, acetaterayon, and bleached wood pulp fibers. The fiber fineness weight of thematerials in the top layer may be in the range of 2.8 to 28 microgramsper inch for the crimped material on the basis of the density ofcellulose and 2.8 to 28 micrograms per inch for the other fibers on sucha basis; and the fibers in the upper part of the bottom layer may have afineness weight in the range of 2.8 to 6 micrograms per inch on such abasis.

The crimps in the fibers may be rough or irregular or regular in aroughly Z-dimensional zone or in a 3-dirnensional zone. The wave lengthsand amplitudes thereof each may be in the range of about 0.01 to 0.33inch. The crimped fibers should have at least three crimps orwave-loops, and this will determine the minimum length thereof.

The weight of the omnidirectional strength layer may be in the range of600 to 1200 grains/sq. yd. and if a top layer is employed, the weight ofthe top layer of 6 carded, crimped rayon may be in the range of 200 to800 grains/ sq; yd. The top or incoming layer may also haveomnidirectional strength instead of being carded.

The polycarboxylic acid may be a polymer of acrylic acid or may beobtained by polymerizing acrylonitrile followed by hydrolysis; oralternatively, it may be a copolymer or a cross-linked polymer such asdivinyl compound copolymerized with an acrylic material. The molecularweight thereof must be at least 90,000 (Staudinger) and may be up toabout 4,000,000.

If lower molecular weight polycarboxylic acid is used the resultingproduct is objectionable because of water extractable material. Highermolecular polycarboxylic acid is objectionable due to higher solutionviscosity which is more difficult to handle and results in loss ofdesired porosity characteristics in the non-woven fabric.

The impregnating bath may contain other ingredients than the polyvinylalcohol and the polyacrylic acid, such as corn starch, polyvinylacetate, glycerine, or hydroxyethylcellulose.

The pH of the impregnating bath may be adjusted so that it is in therange of 3.0 to 5.0. The pH may be adjusted, if too high, by theaddition of phosphoric acid, lactic acid, acetic, citric, glycollic,mineral acids and the like or hydroxide or the like alkaline agent iftoo low.

The drying or setting temperatures for the impregnation may be inf therange of 250 to 400 F., and the times may be in the range of 2 to 4minutes.

The amount of binder in the non-woven fabric on a dry solids basisrelative to the dry weight of the fabric alone may be in the range of1.25 to 10.6%. Lower amounts do not provide boil-proof bonding. Higheramounts might be used, if the higher viscosity thereof is notobjectionable, but they are indicated for economic reasons.

The amount of the polyvinyl alcohol is in the range of 1 to 10 parts perpart by weight of the polycarboxylic acid.

If too low a proportion of polyvinyl alcohol is used, the solutionviscosity is undesirably high and sufficient solids concentration cannotbe obtained to give a boilproof fabric. If too high a proportion ofpolyvinyl alcohol is used, the final product does not have the desiredboil-proof properties even at very high binder add-on.

The adhesive layer used in forming a tube or possibly a bag (tube withone end closed by a single adhesive seam or by double foldingV the endand interlining the folds with adhesive) may be a polyethenyl,polyethylene, or polypropylene or blends thereof having a softeningpoint above about C. Another seam adhesive is polyvinyl acetatecopolymer with small proportion of either dibutyl maleate or dibutylfumarate, as well as a very small amount of acrylic ester if desired.Another seam adhesive is the alcohol polycarboxylic acid of Example 1,but with a thickener such as methyl cellulose in an amount to provide aviscosity of about 20,000 to 50,000 centipoise (as measured on aBrookfield viscometer, Model LVF with a No. 4 spindle at 6 r.p.m.).

Another seam adhesive is a polyvinyl chloride plastisol such as soldcommercially under the name of Bakelite QYNV suspended in acetyltributylcitrate in the proportion range of 1.0 to 1.5 parts of polyvinylchloride to 1 heat such that the plastisol reaches a temperature of 250part of the citrate. The seam must be solidified with to 350 F., whichthen results in a boil-proof seam. Other polyvinyl chlorides orpolyvinyl chloride/acetate copolymers and plasticizers having equivalentproperties may be used.

Instead of the blend of fibers set forth in Example 1, other blends offibers in the fineness range of 2.0 to 25.0 micrograms per inch may beused. The proportions of different fineness fibers should be adjusted togive an average fiber fineness of 6.0 to 12.0 micrograms per inch.

In view of the foregoing disclosures, variations or modificationsthereof will be apparent, and it is intended to include within theinvention all such variations and modifications except as do not comewithin the scope of the appended claims.

We claim:

1. A method of making filter media having omnidirectional strength andgraded porosity which method cornprises assembling a relatively uniform,homogeneous web of fibers including fibers of a lplurality of weightsand sizes, removing fibers from the edge of said web and forceablythrowing said fibers across the path of a rapidly moving air streamhaving a substantially uniform initial fiow, the fibers beingdistributed across said air stream according to their weight and sizethrough the initial throwing force with the lighter fibers traveling theshortest distance across the said -air stream and the heavier fiberstraveling the greatest distance across said air stream and said fibersrbeing carried generally horizontally by said air stream while beingseparated by said initial throwing force, and moving a barrier adjacentsaid air stream and in the paths of said fibers whereby said fibers aredeposited thereon, said fibe-rs being deposited sequentially inaccordance with the weight and size thereof whereby the web n saidbarrier has a graded porosity.

2. The method `of claim 1 wherein said graded web is separated from theair stream and impregnated with a bonding agent.

3. The method of claim 2 'wherein said impregnated web is heated to setsaid bonding agent in said web.

4. The method of claim 1 wherein the graded web is separated from theair stream, impregnated with an aqueous solution containing polyvinylalcohol and a polyacrylic acid and having a pH in the range of about 3to 5, the wet pick-up of the solution being in the range of 200 to 300percent, and heating said web at 250 to 400 F. yfor a period of two tofour minutes.

S. The method of claim 1 wherein said barrier is a foraminous barrierand is moved through said air stream.

I6. The method of claim 5 wherein said fibers are thrown downwardlyacross said air stream and said foraminous barrier is moved upwardlythrough said air stream where- -by the heavier and larger fibers aredeposited on said foraminous barrier first, and lighter and smaller bersare superimposed upon said barrier and said heavier and larger fibers,and the resulting graded web is separated from the air stream andimpregnated with a bonding agent.

7. The method of claim 6 in which threads are included in saidhomogeneous web of fibers said threads being larger and heavier thansaid fibers so that said threads are disposed in a random manner on onesurface of said web and comprise 4a supporting layer.

8. A method of making boil-proof filter media having omnidirectionalstrength and graded porosity which method comprises forming a web of amixture of fibers having the following composition:

20 parts by weight of crimped rayon of about 1.6 inch S staple lengthand about 5.5 denier (15.5 micrograms per inch actual fiber length)diameter, each fiber of which contains 6 to 15 crimps per inch (crimpedbasis) each crimp having an amplitude (height from mean) of about 1/32inch and a wave length of 1A5 to 1/6 inch; 45 parts by weight of similarcrimped rayon of a fineness of 8.5 micrograms per inch; 20 parts byweight of similar crimped rayon of a fineness of about 4.25 microgramsper inch; 15 parts by weight of cotton having a fineness of about 3 to 6micrograms per inch (9.9 to 14 microns diameter average) and an averageclasser staple length of about 0.8 inch; and 20 parts by weight (basedon the weight of the above mixture) of bleached absorbent cotton orrayon threads 0.5 to 3 inches in length, and having a cotton count of40; removing fibers -from the edge of said web and .forceably throwingsaid fibers across the path of a rapidly moving air stream having `asubstantially uniform initial flow, the fibers being distributed acrosssaid air stream according to their weight and size through the initialthrowing force with the lighter fibers traveling the shortest distanceacross said air stream and the heavier fibers traveling the greatestdistance across said air stream and said fibers being carried generallyhorizontally by said air stream while being separated by said initialthrowing force, moving a foraminous barrier across said air stream whilecontinuing the movement of the air whereby said foraminous barrier stopsthe movement of the fibers and said fibers are deposited thereon insuccessive zones according to their weights and sizes, separating theresulting web from the air stream, impregnating said web with an aqueoussolution containing polyvinyl alcohol and a polyacrylic acid and havinga pH in the range of about 3 to 5, the wet pickup of the solution being200 to 300%, and placing it in an oven at 250 to 400 F. for 2 to 4minutes.

References Cited by the Examiner UNITED STATES PATENTS 2,624,079 l/1953Duvall 264-113 XR 2,746,895 5/ 1956 Duvall 264-113 2,769,741 11/1956Schwartz 264-118 3,010,161 11/1961 Duvall 264-115 XR 3,028,287 4/1962Greten 264-121 3,073,735 1/1963 Till et al 264-118 XR FOREIGN PATENTS805,724 12/ 1958 Great Britain.

ALEXANDER H. BRODMERKEL, Primary Examiner.

EARL M. BERGERT, ROBERT F. WHITE, Examiners.

1. A METHOD OF MAKING FILTER MEDIA HAVING OMNIDIRECTIONAL STRENGTH ANDGRADED PORSOITY WHICH METHOD COMPRISES ASSEMBLING A RELATIVELY UNIFORM,HOMOGENEOUS WEB OF FIBERS INCLUDING FIBERS OF A PLURALITY OF WEIGHTS ANDSIZE, REMOVING FIBERS FROM THE EDGE OF SAID WEB AND FORCEABLY THROWINGSAID FIBERS ACROSS THE PATH OF A RAPIDLY MOVING AIR STREAM HAVING ASUBSTANTIALLY UNIFORM INITIAL FLOW, THE FIBERS BEING SIDTRIBUTED ACROSSSAID AIR STREAM ACCORDING TO THEIR WEIGHT AND SIZE THROUGH THE INITIALTHROWING FORCE WITH THE LIGHTER FIBERS TRAVELING THE SHORTEST DISTANCEACROSS THE SAID AIR STREAM AND THE HEAVIER FIBERS TRAVELING THE GREATESTDISTANCE ACROSS SAID AIR STREAM AND SAID FIBERS BEING CARRIED GENERALLYHORIZONTALLY BY SAID AIR STREAM WHILE BEING SEPARATED BY SAID INITIALTHROWING FORCE, AND MOVING A BARRIER ADJACENT SAID AIR STREAM AND IN THEPATHS OF SAID FIBERS WHEREBY SAID FIBERS ARE DEPOSITED THEREON, SAIDFIBERS BEING DEPOSITED SEQUENTIALLY IN ACCORDANCE WITH THE WEIGHT ANDSIZE THEREOF WHEREBY THE WEB ON SAID BARRIER HAS A GRADED POROSITY.